The African trypanosomes in man, domestic or experimental animals produces a variety of well described clinical symptoms, ultimately leading to death. In addition a variety of physiological and immunological changes have been reported. However, the actual cause of these pathological changes is not known. The current hypothesizes to explain the pathology ranges from the production of toxins (both large and small molecular weight substances) to various forms of immunopathology. All of the studies on pathogenesis have utilized animals. Although the mouse has been the preferred model, recent investigations have also utilized rats, rabbits, domestic farm animals, and even man. Other than short-term invitro studies using tissue(s) harvested from animals, there have been no fully invitro models studies. In recent investigations on both drug testing, as well as studies on the mechanism(s) of pathogenesis, we have utilized a completely invitro model. It involved blood trypanosomes grown in the presence of established mammalian cell lines. The host cell that we have routinely utilized is the cloned human promyelocytic HL-60 cell. In this model, the trypanosomes grow readily, but also have been shown to significantly inhibit the growth of the HL-60 cells. It is hypothesized that the trypanosomes produce soluble toxins which are involved in pathogenesis. It is believed that this invitro model will allow us to identify the toxic inhibitors, as well as to investigate their mechanism of action. Finally, this invitro model when fully developed should be an excellent substitute for current primary drug screen using intact animals, since the effect of any drug can be rapidly and simultaneously measured on both the mammalian host cell and the trypanosomes. Our specific aims are: (1) to mimic the invivo state, and to use the invitro model to elucidate specific biochemical and molecular mechanisms responsible for the pathology observed invivo. (2) To develop an invitro model that will simultaneously assess antiparasitic activity as well as the differential toxicity of new chemotherapeutic agents to both host and parasite.